As used herein, the term thyristor refers generically to devices of the four-layer (pnpn) type having three terminals (anode, cathode and gate), and of the five-layer (npnpn) type having three terminals (two anodes and gate). The five-layer type is commonly referred to in the literature as a triac, while the four-layer type is referred to as a silicon controlled rectifier (SCR). These devices are two-state components which operate either in a high-impedance off state or a low-impedance on state; switching from the off to the on state is achieved by the application of a suitable current pulse to the gate.
Both types of devices suffer in that switching from on to off requires first an interruption of the anode current for an interval which exceeds a minimum time span (t.sub.q) followed by a reapplication of anode voltage which does not exceed a critical rate of rise (dv/dt). If either the t.sub.q or the dv/dt condition is not met, switching to the off state may fail to occur.
While thyristor devices suffer the above problems associated with turn-off, they are nevertheless attractive in terms of cost, reliability and ruggedness.
In certain applications where the input power is ac, such as phase-controlled bridges and light dimmers, the thyristors turn-off characteristics are not problematic because turn-off is inherently provided by the reversals in the input line voltage which occur each half cycle. However, in most other applications where the input power is dc, such as inverters and choppers, additional circuitry must be used to provide commutation. Examples of such external commutation circuitry range from schemes involving capacitors, inductors, diodes and commutating SCR's to circuits using transistors and commutating power supplies. Typically, the cost, weight and power losses associated with these commutating components are dominant over the cost, weight and losses directly associated with the thyristors themselves. It is for this reason that bijunction transistors have supplanted thyristors in many inverter and chopper applications where high current rating is not required for switches.